Waste heat recovery turbine system

ABSTRACT

Provided is a waste heat recovery turbine system capable of lowering the temperature of hot water returned to a heat source and of suppressing a steaming phenomenon in a preheater without changing the flow rate of a heating medium flowing through an evaporator. The waste heat recovery turbine system comprises a turbine  4  driven by a working medium M, an evaporator  10  which evaporates the working medium M by heat exchange with a heating medium H supplied from a first heat source  20   a  and supplies the evaporated working medium M to the turbine  4 , a preheater  12  for preheating the working medium M flowing into the evaporator  10 , a preheating passage  22   b  through which the heating medium H is supplied from the evaporator  10  to the preheater  12 , a return passage  22   d  which branches from a location of the preheating passage  22   b  and serves to return the heating medium H to a second heat source  20   b , and a flow control valve  24  for controlling a flow rate of the heating medium H supplied to the preheater  12  by causing the heating medium H which has radiated heat in the evaporator  10  to flow through the return passage  22   d.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to a waste heat recovery turbine system which uses a medium other than water, in particular, a medium with a lower boiling point than the water.

2. Background Art

In general, in a waste heat recovery turbine system, as shown in FIG. 7, a heating medium H such as heated water or hot water from a heat source 100 is utilized, an evaporator 101 evaporates a working medium M with a low boiling point by heat exchange with the heating medium H, and the working medium M in a vapor phase drives a turbine 102. The heating medium H flows out from the evaporator 101. Further, a preheater 103 exchanges heat of the heating medium H with the working medium M, and thereafter the heating medium H is returned to the heat source 100. The flow rate of the hot water H flowing through the evaporator 101 and the flow rate of the hot water H flowing through the preheater 103 are controlled by a flow control valve 104 provided upstream of the evaporator 101 (patent literature 1).

-   [Patent literature 1] Japanese Laid-Open Patent Application     Publication No. Hei. 5-222906

SUMMARY OF THE INVENTION

In the waste heat recovery turbine system, it is desirable to decrease the temperature of the heating medium H returned to the heat source to one as low as possible to recover more waste heat from the heat source. However, in the above system, since the flow rate of the heating medium H flowing through the evaporator 101 is equal to the flow rate of the heating medium H flowing through the preheater 103, it is difficult to conduct heat exchange in a well-balanced manner in the evaporator 101 and the preheater 103. Therefore, it is difficult to control the temperature of the heating medium H returned to the heat source 100. If the amount of the heating medium H flowing into the preheater 103 is reduced to avoid so-called a steaming phenomenon that the working medium M is evaporated in the preheater 103 due to a fluctuation in the flow rate or temperature of the heating medium H flowing through the preheater 103, the flow rate of the heating medium H flowing through the evaporator 101 is also reduced. This reduces a heat exchange amount in the evaporator 101.

The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a waste heat recovery turbine system which is capable of lowering the temperature of the heating medium returned to the heat source and suppressing the steaming phenomenon in the preheater without changing the flow rate of the heating medium flowing through the evaporator.

To achieve the above mentioned object, a waste heat recovery turbine system of the present invention, comprises a turbine driven by a working medium; an evaporator which evaporates the working medium by heat exchange with a heating medium supplied from a heat source and supplies the evaporated working medium to the turbine; a preheater for preheating the working medium flowing into the evaporator; a preheating passage through which the heating medium is supplied from the evaporator to the preheater; a return passage which branches from a location of the preheating passage and serves to return the heating medium to the heat source; and a flow control mechanism for controlling a flow rate of the heating medium supplied to the preheater by causing the heating medium which has radiated heat in the evaporator to flow through the return passage.

In accordance with this configuration, since the flow control mechanism causes a part of the heating medium which has radiated heat in the evaporator to flow through the return passage, the flow rate of the heating medium supplied to the preheater can be lessened, and therefore, the temperature of the hot water returned from the preheater to the heat source can be lowered. The hot water with a low temperature can be re-used for cooling. In addition, since the flow control mechanism is positioned downstream of the evaporator, a steaming phenomenon in the preheater can be suppressed without reducing the flow rate of the heating medium flowing through the evaporator, i.e., without reducing the output in a cycle.

In the present invention, the flow control mechanism is preferably provided in the return passage. In accordance with this configuration, the flow control mechanism provided in the return passage is closed to inhibit the heating medium from flowing through the return passage, in a case where it is necessary to supply all of the heating medium which has flowed through the evaporator, to the preheater, and thus, all of the heating medium which has flowed through the evaporator can be supplied to the preheater. In that case, since the heating medium does not flow through the flow control mechanism, a pressure loss due to the flow control mechanism will not be generated within the preheating passage.

In the present invention, the flow control mechanism may be provided in the preheating passage, and the return passage may branch from the preheating passage in a location upstream of the flow control mechanism. In accordance with this configuration, the flow rate of the heating medium supplied to the preheater can be controlled more correctly, and the temperature of the hot water returned from the preheater to the heat source can be controlled precisely.

In the present invention, the waste heat recovery turbine system may preferably further comprise a temperature sensor for detecting a temperature of the working medium at an outlet of the preheater; and a temperature control means configured to control the flow control mechanism so that the detected temperature does not exceed a predetermined value. In accordance with this configuration, since the temperature of the working medium inside the preheater can be controlled, the steaming phenomenon in the preheater can be suppressed efficiently.

In a preferred embodiment of the present invention, the preheater includes a shell and tubes inserted into an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell. Typically, the heating medium is flowed through the tube and the working medium is flowed through the interior of the shell to prevent a vapor from causing a vapor lock in a passage of the working medium even if the steaming occurs. In contrast, in this embodiment, the working medium is flowed through the tube and the heating medium is flowed through the interior of the shell, because occurrence of the steaming can be prevented in the present invention. This makes it possible to lessen the amount of the working medium.

In accordance with the present invention, since the flow control mechanism causes a part of the heating medium which has radiated heat in the evaporator to flow through the return passage, the flow rate of the heating medium supplied to the preheater can be lessened, and therefore, the temperature of the hot water returned from the preheater to the heat source can be lowered. In addition, the steaming phenomenon in the preheater can be suppressed without reducing the flow rate of the heating medium flowing through the evaporator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system view of a turbine power generation system including a waste heat recovery turbine system according to Embodiment 1 of the present invention.

FIG. 2 is a cross-sectional view of a preheater in the turbine power generation system.

FIG. 3 is a system view of a hot water circulating passage in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 2 of the present invention.

FIG. 4 is a system view of a hot water circulating passage in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 3 of the present invention.

FIG. 5 is a system view of a hot water circulating passage in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 4 of the present invention.

FIG. 6 is a system view of a hot water circulating passage in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 5 of the present invention.

FIG. 7 is a system view of a conventional turbine power generation system including a waste heat recovery turbine system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a system view of a turbine power generation system including a waste heat recovery turbine system according to Embodiment 1 of the present invention. A turbine power generation system 1 includes a turbine power generation unit 6 including a generator 2 and a turbine 4 for driving the generator 2. The turbine power generation system 1 further includes on a medium passage 8 through which a working medium M for the turbine 4 is circulated, an evaporator 10 of a heat transfer tube type, a preheater 12, a working medium feed pump 14, a condensed liquid tank 16 and a condenser 18. The condensed liquid tank 16 may be omitted.

The evaporator 10 is configured to evaporate the working medium M by utilizing heat energy of the heating medium H supplied from a heat source 20 by heat exchange and supply the working medium M in a vapor phase to the turbine power generation unit 6 via a vapor phase medium feeding passage 8 a. After driving the turbine 4 in the turbine power generation unit 6, the working medium M is fed to the condenser 18 via a vapor phase medium recovery passage 8 b. The working medium M is liquefied in the condenser 18, and then is supplied to the evaporator 10 after its pressure is raised by a working medium feed pump 14 attached to a liquid phase medium feeding passage 8 c. On the liquid phase medium feeding passage 8 c, the condensed liquid tank 16 for stabilizing a suction pressure of the working medium feed pump 14 is provided upstream of the working medium feed pump 14, and the preheater 12 for preheating the working medium M flowing into the evaporator 10 is provided upstream of the evaporator 10 and downstream of the working medium feed pump 14. The medium passage 8, which is a circulating passage, is constituted by the vapor phase medium feeding passage 8 a, the vapor phase medium recovery passage 8 b and the liquid phase medium feeding passage 8 c. Since the working medium M is circulated within a space sealed from atmosphere, including the medium passage 8, the preheater 12, the evaporator 10, the turbine 4 and the condenser 18, it is possible to prevent the working medium M from flowing out to atmosphere.

A first temperature sensor T1 and a second temperature sensor T2 are attached on the outlet of the evaporator 10 and the outlet of the preheater 12 on the liquid phase medium feeding passage 8 c, respectively. The first temperature sensor T1 may be omitted.

The working medium M is a mixture of a main medium with a low boiling point and a lubricant. As the main medium, there are HFE (hydrofluoroether), i.e., substances which are obtained by substituting a part of H with F in a general expression C_(n)H_(2n+1)—O—C_(m)H_(2m+1), have boiling points higher than 25 degrees C. and lower than 100 degrees C. in a normal pressure, and contain carbons C which are not more than seven in number, for example, C₃F₇OCH₃, C₄F₉OCH₃, C₄F₉OC₂H₅, C₆F₁₃OCH₃ and CHF₂—CF₂—O—CH₂—CF₃. Among these, a specific example of C₃F₇OCH₃ is available from SUMITOMO 3M under the trade name of Novec 7000. As other alternative media, there are HFC (hydrofluorocarbon) obtained by substituting a part of H with F in C_(n)H_(2n+2), FE (fluoroether) obtained by substituting all of H with F in a general expression C_(n)H_(2n+1)—O—C_(m)H_(2m+1), and fluorinated alcohol obtained by substituting a part of H other than OH with F in C_(n)H_(2n+1)—OH.

The reason why the medium represented by the above HFE (hydrofluoroether) is suitable for use as the medium in the turbine power generation system is that it does not deplete an ozone layer because ozone depletion potential ODP=0, has low global warming potential GWP, and has excellent environmental friendliness. For example, the global warming potential GWP of the Novec 7000 is 370. As other medium having excellent environmental friendliness, like HFE, there is HFO (hydrofluoroolefin) which may also be used as the main medium. Furthermore, so-called natural medium such as ammonia, butane, or pentane may be used as the main medium.

The condenser 18 has a known structure, including tubes of a cooling medium C inserted into the interior thereof. The condenser 18 is configured to cool and liquefy the working medium M in a vapor phase using the cooling medium C, after the working medium M has driven the turbine 4.

The heat source 20 is, for example, waste heat such as hot water generated in manufacturing processes in iron mills, ceramic engineering, etc, and includes heat sources with a plurality of temperatures, which are for example, a first heat source 20 a containing hot water of 98 degrees C., a second heat source 20 b containing hot water with a temperature slightly higher than 87 degrees C., and a third heat source 20 c containing hot water with a temperature slightly higher than 77 degrees C. The hot water H which is a heating medium containing waste heat from the first heat source 20 a is introduced into the heat transfer tubes in the interior of the evaporator 10 through a heating medium feeding passage 22 a, and then is introduced into the preheater 12 through a preheating passage 22 b. A flow control valve for controlling the flow rate of the hot water H introduced into the evaporator 10 may be provided on the heating medium feeding passage 22 a in a location upstream of the evaporator 10, as in the conventional configuration shown in FIG. 7.

As shown in FIG. 2, the preheater 12 includes a shell 12 a and tubes 12 b inserted into the interior of the shell 12 a. The working medium M is flowed through the tubes 12 b and the hot water H supplied from the evaporator 10 is flowed through the interior of the shell 12 a. The hot water H which has flowed through the interior of the shell 12 a of the preheater 12, is returned to the third heat source 20 c through a heating medium recovery passage 22 c of FIG. 1. A flow control valve 24 is provided in a location of the preheating passage 22 b as a flow control mechanism for controlling the flow rate of the hot water H introduced into the preheater 12.

A return passage 22 d branches from the preheating passage 22 b in a location upstream of the flow control valve 24 to return the hot water H to the second heat source 20 b. A part of the hot water H which has radiated heat in the evaporator 10, i.e., the remaining hot water H which has been controlled by the flow control valve 24, flows through the return passage 22 d. A heating medium circulating passage 22, which works to recover the hot water supplied from the first heat source 20 a to the second and third heat sources 20 b and 20 c, is constituted by the heating medium feeding passage 22 a, the preheating passage 22 b, the heating medium recovery passage 22 c, and the return passage 22 d.

The flow control valve 24 is controlled by a temperature control means 30. The outputs of the first and second temperature sensors T1 and T2 are input to the temperature control means 30. In other words, the flow control valve 24 is controlled based on the outputs of the first and second temperature sensors T1 and T2. To be specific, when the temperature detected by the first temperature sensor T1 is t1 and the temperature detected by the second temperature sensor T2 is t2, the flow control valve 24 is controlled so that t1 is higher than t2 by a certain value or larger, preferably t1−t2 is equal to +5˜8 degrees C. If the first temperature sensor T1 is omitted, the flow control valve 24 is controlled so that a difference between a boiling point v1 of the working medium M and the temperature t2 detected by the second temperature sensor T2 is the above value.

The operation of the above system will be described with reference to FIG. 1. Firstly, the operation of the working medium M will be described. The system of FIG. 1 uses the working medium M comprising HFE with a boiling point of 34 degrees C. in a normal pressure (1 atmospheric pressure). The hot water H with 98 degrees C. which has been derived from the first heat source 20 a is introduced into the evaporator 10 through the heating medium feeding passage 22 a. In the evaporator 10, the introduced working medium M exchanges heat with the introduced hot water H, i.e., receives heat from the hot water H and is evaporated into a high-pressure vapor phase with 80 degrees C. and 415 kPaA (absolute pressure).

The working fluid M converted into a vapor phase is taken out from the upper portion of the evaporator 10, is supplied to the turbine 4 in the turbine power generation unit 6 through the vapor phase medium feeding passage 8 a, and drives the turbine 4. Thereupon, the generator 2 coupled to the turbine 4 by a rotary shaft is driven to generate electric power. The working medium M which has released energy in the turbine 4 decreases its temperature to 57 degrees C. and its pressure to 88 kPaA. The working medium M flows into the condenser 18 through the vapor phase medium recovery passage 8 b and is cooled and liquefied by heat exchange with the cooling medium C with 20 degrees C. in the condenser 18. After converted into a liquid phase with 30 degrees C. in this way, the working medium M is pressurized by the working medium feed pump 14 while flowing through the liquid phase medium feeding passage 8 c, supplied to the preheater 12, and preheated up to 72 degrees C. by heat exchange with the hot water H with 87 degrees C. which has flowed through the evaporator 10, inside the preheater 12. Thereafter, the working medium M is returned to the evaporator 10.

Next, the operation of the hot water H which is the heating medium will be described. The hot water H with 98 degrees C. is introduced from the first heat source 20 a into the evaporator 10 through the heating medium feeding passage 22 a, and exchanges heat with the working medium M in the evaporator 10 to decrease its temperature to 87 degrees C. The hot water H with 87 degrees C. is introduced into the preheater 12 through the preheating passage 22 b, with a flow rate controlled by the flow control valve 24, while the remaining hot water H is returned to the second heat source 20 b through the return passage 22 d. The hot water H which has been introduced into the preheater 12, exchanges heat with the working medium M, decreases its temperature to 77 degrees C., and then is returned to the third heat source 20 c through the heating medium recovery passage 22 c.

In the conventional example of FIG. 7, if the working medium M comprising HFE with a boiling point of 34 degrees C. in a normal pressure (1 atmospheric pressure) and the hot water H with 98 degrees C. as the heating medium, are used, like Embodiment 1 shown in FIG. 1, the temperature of the hot water H returned to the heat source 100 is 83 degrees C., which is a relatively high temperature. On the other hand, in Embodiment 1 shown in FIG. 1, the temperature of the hot water H recovered by the third heat source 20 c can be lowered to 77 degrees C.

In the above configuration, since the flow control valve 24 causes a part of the hot water H which has radiated heat in the evaporator 10 to flow through the return passage 22 d, the flow rate of the hot water H supplied to the preheater 12 is lessened, and therefore the temperature of the hot water H returned from the preheater 12 to the heat source 20 can be lowered. For example, the hot water H lowered in temperature can be re-used to cool an object to be cooled, such as a converter furnace. When the flow control valve 24 is fully opened, the whole amount of the hot water H can be supplied to the preheater 12, while when the flow control valve 24 is fully closed, the whole amount of the hot water H is supplied to the return passage 22 d. Since the flow control valve 24 is positioned downstream of the evaporator 10, it is possible to suppress a steaming phenomenon in the preheater 12 without reducing the flow rate of the hot water H flowing through the evaporator 10, i.e., reducing an output in a cycle.

Furthermore, since the flow control valve 24 is provided in the preheating passage 22 b and the return passage 22 d branches from the preheating passage 22 b in a location upstream of the flow control valve 24, the flow rate of the hot water H supplied to the preheater 12 can be controlled more correctly, and the temperature of the hot water H returned from the preheater 12 to the heat source 20 can be controlled precisely.

Since the first temperature sensor T1 and the second temperature sensor T2 are provided to detect the temperature of the working medium M at the outlet of the evaporator 10 and the temperature of the working medium M at the outlet of the preheater 12, respectively, and the temperature control means 30 controls the flow control valve 24 so that a difference between the detected temperature t1 and the detected temperature t2 does not exceed a predetermined value, the temperature of working medium M inside the preheater 12 can be controlled. As a result, the steaming phenomenon in the preheater 12 can be suppressed efficiently.

Typically, the heating medium is flowed through the tubes and the working medium is flowed through the interior of the shell to prevent a vapor from causing a vapor lock in a passage of the working medium M even if the steaming occurs. In contrast, in the above configuration, occurrence of the steaming can be prevented, and therefore, the working medium M is flowed through the tubes 12 b and the hot water H is flowed through the interior of the shell 12 a. This makes it possible to lessen the amount of the working medium M, which leads to contribution to prevention of depletion of the ozone layer, prevention of warming, etc.

FIG. 3 is a system view of a heating medium circulating passage 22 in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 2. The constituents other than the heating medium circulating passage 22, which are not shown, are identical to those of Embodiment 1. In this embodiment, the flow control valve 24 is provided on the return passage 22 d and configured to control the flow rate of the hot water H flowing through the return passage 22 d, thereby indirectly controlling the flow rate of the hot water H supplied to the preheater 12. In accordance with this embodiment, in a case where it is necessary to supply all of the hot water H which has flowed through the evaporator 10, to the preheater 12, the flow control valve 24 provided in the return passage 22 d is closed to inhibit the hot water H from flowing through the return passage 22 d so that all of the hot water H which has flowed through the evaporator 10 can be supplied to the preheater 12. In this case, the hot water H does not flow through the flow control valve 24, and therefore a pressure loss due to the flow control valve 24 is not generated within the preheating passage 22 b. If a pressure loss in the heating medium circulating passage 22 is not problematic, then the flow control valve 24 may be provided in the preheating passage 22 b to directly control the hot water H supplied to the preheater 12 as shown in FIG. 1.

FIG. 4 is a system view of a heating medium circulating passage 22 in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 3. In this embodiment, a three-way valve 26 is provided as a flow control mechanism in a location where the return passage 22 d branches from the preheating passage 22 b, and configured to control the flow rate of the hot water H flowing through the preheating passage 22 b and the flow rate of the hot water H flowing through the return passage 22 d based on a signal from the temperature control means 30 (FIG. 1). In this embodiment, the same advantages as those of Embodiment 1 are achieved.

FIG. 5 is a system view of a heating medium circulating passage 22 in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 4. In this embodiment, a variable displacement pump 28 is provided in the preheating passage 22 b as a flow control mechanism, and the return passage 22 d branches from the preheating passage 22 b in a location upstream of the variable displacement pump 28. The number of rotations of a motor of the variable displacement pump 28 is controlled based on a signal from the temperature control means 30 (FIG. 1) to control the flow rate of the hot water H supplied to the preheater 12. In this embodiment, the same advantages as those of Embodiment 1 are achieved.

FIG. 6 is a system view of a heating medium circulating passage 22 in a turbine power generation system including a waste heat recovery turbine system according to Embodiment 5. In this embodiment, the variable displacement pump 28 is provided in the return passage 22 d as a flow rate control mechanism and configured to control the flow rate of the hot water H flowing through the return passage 22 d, thereby indirectly controlling the flow rate of the hot water H supplied to the preheater 12. In this embodiment, the same advantages as those of Embodiment 2 are achieved.

Although preferred embodiments have been described thus far with reference to the drawings, various additions, alternations or deletions can be made without departing from the concept of invention, and therefore such additions, alternations or deletions should be construed as being within the scope of the present invention.

REFERENCE SIGNS LIST

-   4 turbine -   10 evaporator -   12 preheater -   12 a shell -   12 b tube -   20 heat source -   22 b preheating passage -   22 d return passage -   24 flow control valve (flow control mechanism) -   26 three-way valve (flow control mechanism) -   28 variable displacement pump (flow control mechanism) -   30 temperature control means -   H heating medium (hot water) -   M working medium -   T1 first temperature sensor -   T2 second temperature sensor -   C cooling medium 

1. A waste heat recovery turbine system comprising: a turbine driven by a working medium; an evaporator which evaporates the working medium by heat exchange with a heating medium supplied from a heat source and supplies the evaporated working medium to the turbine; a preheater for preheating the working medium flowing into the evaporator; a preheating passage through which the heating medium is supplied from the evaporator to the preheater; a return passage which branches from a location of the preheating passage and serves to return the heating medium to the heat source; and a flow control mechanism for controlling a flow rate of the heating medium supplied to the preheater by causing the heating medium which has radiated heat in the evaporator to flow through the return passage.
 2. The waste heat recovery turbine system according to claim 1, wherein the flow control mechanism is provided in the return passage.
 3. The waste heat recovery turbine system according to claim 1, wherein the flow control mechanism is provided in the preheating passage, and the return passage branches from the preheating passage in a location upstream of the flow control mechanism.
 4. The waste heat recovery turbine system according to claim 1, further comprising: a temperature sensor for detecting a temperature of the working medium at an outlet of the preheater; and a temperature control means configured to control the flow control mechanism so that the detected temperature does not exceed a predetermined value.
 5. The waste heat recovery turbine system according to claim 1, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell.
 6. The waste heat recovery turbine system according to claim 2, further comprising: a temperature sensor for detecting a temperature of the working medium at an outlet of the preheater; and a temperature control means configured to control the flow control mechanism so that the detected temperature does not exceed a predetermined value.
 7. The waste heat recovery turbine system according to claim 3, further comprising: a temperature sensor for detecting a temperature of the working medium at an outlet of the preheater; and a temperature control means configured to control the flow control mechanism so that the detected temperature does not exceed a predetermined value.
 8. The waste heat recovery turbine system according to claim 2, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell.
 9. The waste heat recovery turbine system according to claim 3, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell.
 10. The waste heat recovery turbine system according to claim 4, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell.
 11. The waste heat recovery turbine system according to claim 6, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell.
 12. The waste heat recovery turbine system according to claim 7, wherein the preheater includes a shell and tubes inserted through an interior of the shell, the working medium being flowed through the tubes and the heating medium being flowed through the interior of the shell. 